Generally, a suspension of a vehicle has a subframe located between left and right wheels. While being coupled with the vehicle body, this subframe is coupled with the wheels through lower arms and knuckles. Therefore, the subframe is required to have sufficient rigidity against forces transmitted thereto from the wheels and acting thereon in the front-rear, left-right, and top-bottom directions. Further, it is preferable that the subframe be configured to efficiently transmit and reliably support these inputs.
As a conventional subframe, a subframe obtained by press-molding a relatively large plate material is used. For the purpose of imparting high rigidity to this subframe, for example, Japanese Patent No. 2963749 discloses one having irregularities partially formed on the plate material, whereas Japanese Unexamined Patent Application Publication No. 2005-138652 discloses one having a reinforcement body provided between an upper panel and a lower panel.
However, the use of a plate material having such a large area is likely to cause wastes from a parent material when a material for a subframe is cut out from the parent material in the manufacture thereof, and there is a risk of manufacturing the subframe at a cost disadvantage. There are cases configured for the purpose of imparting high rigidity to a subframe, such as one where a subframe has irregularities partially formed thereon, and one where a subframe has a reinforcement body inside an upper panel and a lower panel. However, neither of these cases is preferable because the weight of the whole subframe increases as a result. Particularly, in the case intended to secure rigidity by forming irregularities on a plate material, it is required that the number of times for press work on the plate material is increased, which complicates the manufacture of the subframe and manufactures the subframe at a cost disadvantage.